A year in the life of Mars.

Humans have journeyed to Mars for generations. Jetting to the Red Planet on high-octane imagination, novelists have spun tales of parched Martian cities, bug-eyed monsters, and an ancient race of canal builders.

Eventually, technology began to replace daydreams as the route to Mars. In 1964, the Mariner 4 space probe revealed the planet's true face: barren, heavily cratered, bereft of breathable air - and clearly uninhabited by the tentacled warriors and inscrutable aliens described in science-fiction stories of previous decades.

Other U.S. spacecraft followed the trail blazed by Mariner 4, culminating in 1976 with the triumphant touchdown of two robot laboratories called the Viking Landers. After a series of largely unsuccessful Soviet attempts to explore Mars, Russian space scientists in 1988 sent a pair of probes to Phobos, one of the Red Planet's two moons. These ill-fated missions were cut short by technical snafus and operator errors.

Now, for the first time in 17 years, a U.S. spacecraft is again hurtling toward Mars (SN: 9/19/92, p. 181). The Mars Observer will reach its destination on Aug. 24. After a three-month respite, during which the craft will gradually assume a circular, nearly pole-crossing orbit, mission controllers will turn on the orbiting observatory's instruments. At that moment -- more than 13 years after the radio transmitter of the last surviving Viking robot went silent -- NASA's network of giant ground-based antennas will once again hear the drone of scientific data from a U.S. visitor to Mars.

Mars Observer will probe the planet's surface and atmosphere for at least an entire Martian year, or 687 Earth days. If the craft's batteries and control rockets hold out, the mission could continue to the turn of the century. The information beamed home to Earth -- pouring in from space fast enough to fill a computer's 120-megabyte hard drive every day -- will offer planetary scientists the most comprehensive portrait of the Red Planet to date, says project scientist Arden L. Albee of the California Institute of Technology in Pasadena.

Scientists are far better prepared to explore the Red Planet now than they were in the early 1960s, when some Mars enthusiasts still hoped to find shallow lakes and vegetation there. Indeed, 114 veteran Mars scientists published a scientific magnum opus on the Red Planet last year, a 1,498-page behemoth appropriately titled Mars (1992, University of Arizona Press). Yet despite the impressive body of observation, theory, and experiment this book represents, a number of important questions about the planet remain unanswered.

Naturally, there are as many big questions about Mars as there are planetary scientists. But a few come up more frequently than others. What processes have sculpted the planet's surface and interior? How does chemical composition vary across the surface? Does Mars have active volcanoes and a magnetic field? To what extent do dust storms affect the weather and climate? How much water is locked up in the Martian soil and atmosphere? And perhaps most intriguing, did Mars ever have an Earth-like climate capable of nurturing life?

While Mars Observer won't clear up all these mysteries, it will at least offer a way to test some of the prevailing explanations for Martian weather, climate, and geology. "With the data from Mariner and Viking, we have ]tentative[ answers for a lot of the big questions about Mars, but those answers are built upon one data set," says Albee. "It's like doing a weather prediction based on last week's weather, but you don't have another week's weather to check it against."

In all likelihood, Mars Observer will slay some beautiful old theories with ugly new facts, to paraphrase the words of 19th-century biologist Thomas H. Huxley. One likely victim will be computer models of the Martian atmosphere, says planetary scientist James B. Pollack of NASA's Ames Research Center in Mountain View, Calif.

Pollack has spent much of the past 20 years helping to create a global circulation model (GCM) for Mars. Built on a foundation of Mariner and Viking observation, his GCM rests on mathematical simulations of the physical processes that drive weather phenomena -- the movement of high- and low-pressure systems, for example.

Mars GCMs predict what the average weather conditions should be on the planet at particular locations and times of the year. For example, before the Viking Landers descended from orbit, Pollack and his colleagues used a GCM to predict the wind conditions on the plains of Chryse (pronounced CRY-see) and Utopia, where the robot laboratories touched down.

Planetary scientists are eager to test their GCMs against Mars Observer's harvest of new data. "In science you never believe a model entirely," says Pollack. "You always want to put it to as many tests as possible." It's very likely, he says, that Mars Observer will turn up unforeseen mismatches between observed Martian weather and the GCM predictions. "I'd almost be disappointed if it didn't."

Mars Observer -- whose main body consists of a rectangular box the size of a large office desk with scientific instruments bolted to its underbelly -- will reach its destination well equipped to continue the work of past explorers. The power of its instruments, says Albee, stems from their synergistic nature: Various sensors and instruments will probe the Martian surface and atmosphere in different regions of the electromagnetic spectrum, returning different kinds of information about the same target areas. "No single instrument does everything," Albee says. "The combination is designed to help us understand a whole host of things."

The orbiting observatory carries gamma-ray detectors to identify the various elements that make up Mars' redtinted soil. The craft will also use radio beams and infrared sensors to study weather systems, dust storms, and currents of water vapor as they flow through the planet's thin, carbon dioxide atmosphere. Mars Observer will search for a magnetic field or evidence of now-extinct magnetism preserved in Martian surface rock. Mission scientists will also map the planet's gravitational field and create precise topographical maps of its fractured, wind-carved surface.

And Mars Observer's pole-crossing orbital path offers some important advantages over past missions. Completing an orbit every 118 minutes as the planet spins beneath it, the craft will achieve uniform, global coverage of the surface every 26 days. In contrast, the Viking Orbiters circled the girth of Mars like a pair of extra moons, building up a patch-work of images captured at different times during the year and under varying lighting conditions.

Mars Observer will also send home a "nightly newscast" of the entire atmosphere, similar to the satellite images of Earth's weather systems shown during television news, Albee says. These daily snapshots may permit planetary scientists to observe the birth and growth of giant dust storms, which play an important role in Martian weather (see sidebar, p.106).

For this global photo shoot, Mars Observer carries two kinds of cameras. One captures wide swaths of the planet's surface in moderate detail, enabling researchers to see features as small as 250 meters across. The other camera scans narrower strips but in much greater detail Together, the cameras will complete the visual exploration of Mars begun by the Viking Orbiters, which achieved detailed mapping of only 15 percent of the planet's surface.

Mission scientists will use the high-resolution camera selectively to map potentially important surface features, such as ancient stream channels. This sharp-eyed instrument can distinguish objects as small as 1.4 meters across-- about the size of a Viking Lander.

With the high-resolution camera, mission scientists will be able to detect changes in the appearance of the soil caused by permafrost, says Albee. On Earth, permafrost creates distinctive patterns that are visible from space. If the same is true on Mars, scientists could use the high-resolution camera to measure how far the Martian permafrost--thought to be a major repository of water in Martian soil -- extends from the poles.

This kind of measurement can help resolve a long-standing puzzle: Where's all the water that once carved huge channels in the surface of Mars? Is it locked up in the soil, or did it somehow escape into space?

"We assume there's a lot of water tied up," says Albee, "but we don't really know that yet."

What about the most provocative of the big questions about Mars: Could it have supported life in the distant past? To some extent, this is the wrong question to ask Mars Observer, whose instruments and experiments are designed to document a year in the life of the planet as it exists today, not as it was 2 billion years ago. But Pollack thinks the present is a good place to begin a scientific journey into the past. "You have to understand the present before you can hope to understand the past," he says. "And in that sense, I think Mars Observer will do a first-rate job."

Mars Observer's focus on the present will leave the mysteries of Martian history for future missions to solve. But Mars will not lack for such visits over the next decade. The space agencies of Russia, Japan, the European Community, and the United States have all announced plans to send orbiters, landers, surface crawlers, and other robotic probes to Mars in coming years.

In many ways, says Pollack, Mars Observer will serve as a reconnaissance mission that will scope out interesting sites for future exploration. For example, its instruments might detect areas rich in carbonate -- possibly the mineral remenant of ancient lakebeds (SN: 6/5/93, p.357). A lander or robot space buggy could then explore such a place up close.

Russian space scientists will spear-head the coming robot invasion of the Red Planet. In 1994, they plan to launch a craft that will deploy surface probes and landers. In 1996, researchers from Russia and France will watch over a second mission that will deploy additional robot explorers on the surface.

NASA's proposed Mars Environmental Survey (MESUR) will include a network of a dozen or more landers. These squat, automated laboratories will monitor the local weather, sample the soil, measure seismic rumblings, and possibly let loose a pack of desktop-sized robot crawlers. NASA hopes to launch MESUR Path-finder, the first lander in the network, in 1996.

When this next clutch of robot explorers awakens on the surface of Mars, scientists will get a much clearer and more comprehensive portrait of the planet's current state, and that could provide clues to what Mars looked like in the distant past.

Such knowledge may also enrich our understanding of other planets in the solar system, says Albee.

"If you look at Mars, Venus, and Earth, the sizes are not too different and they're not too far apart in the solar system," he explains. "The challenge is to understand a lot alike, differ in some very significant ways."